The invention is broadly directed to optical transmission systems utilising optical amplifiers. The invention has particular relevance to output power stabilisation in broadband optical amplifiers.
In optical communication systems, the limited dynamic range and sensitivity of optical receivers imposes certain requirements on the power accuracy of traffic channels. Optical amplifiers, a term which includes Erbium doped fibre amplifiers, fluoride doped fibre amplifiers, Erbium Ytterbium amplifiers, Raman amplifiers, Brillouin amplifiers, semiconductor amplifiers, and the like, are now widely used in such communication systems. A problem common to this class of amplifier is the dependence of gain on the input power. In wavelength division multiplexed (WDM) networks, this implies that for a fixed pump power, the amplifier gain is a function of the number of channels passing through the amplifier. Thus in reconfigurable networks, such as networks using the dynamic addition or drop of optical channels, the gain experienced by any one traffic channel will be affected by the configuration of the network as a whole. While slow power variations due to ageing and improper fibre handling are normally mitigated by relatively slow control feedback loops, transient suppression during network reconfiguration requires relatively fast control of the amplifier gain.
One method that is presently implemented to combat this problem is the use of a feed forward loop for active gain control. In such an arrangement, power input to the amplifier is measured and used to control the power of the amplifier pump. If the input signal power changes, the pump power injected into the amplifier is adjusted to keep the gain unchanged.
However, a characteristic of many optical amplifiers used in broadband systems such as WDM communication systems is that the gain varies depending on signal wavelength. Specifically, some input wavelengths may experience different gains and have a correspondingly different impact on the feed forward loop. This effect stems from the level of stimulated emission decay of excited states being higher when the input is at one wavelength than at another wavelength. The different levels of decay may also depend on the pumping levels. For example, in an Erbium doped fibre amplifier at high pumping levels, i.e. at a high level of population inversion, input wavelengths at around 1530 nm will provoke a higher decay rate and thus a higher gain than those at around 1550 nm at equal input powers. Thus for the same input powers a higher pump power must be applied in order to sustain this level of population inversion when the input signal is at 1530 nm. For the same amplifier at lower pumping levels, wavelengths around 1550 nm may experience a higher gain. Thus in this case for equal input powers a higher pump power would be required to sustain a steady population inversion for an input signal at 1550 nm. For different amplifiers and different pumping levels, a curve of the decay of stimulated emission of the excited population against wavelength may show several peaks centred around different wavelengths. The traffic channels on the link will thus experience varying signal power.
It is thus an object of the invention to provide an optical amplifier arrangement having stabile gain for signals over broad spectral range and that is substantially insensitive to rapid changes in network configuration.
According to the invention, there is provided an amplifier arrangement with a feed-forward gain-control loop with a weighting arrangement capable of weighting the signal power of selected wavelengths received by the amplifier. In this way the non-uniform spectral gain of the amplifier may be compensated for by appropriate selection of the weighting factor. Hence the influence of wavelengths having a greater than average impact on the decay rate of the excited population can be increased and similarly the influence of those wavelengths that have a smaller than average impact on decay rate can be reduced. By tailoring the weighting arrangement to the specific class of amplifier utilised, and possibly to a specific amplifier within a class, amplifier gain can be precisely controlled and traffic signal power reliably held at a constant level regardless of link configuration changes.
The weighting may be performed in a single step using a filter with a transfer function designed to vary with wavelength, such that the input power in predetermined wavelength bands is attenuated or amplified as required.
In an alternative arrangement, the extracted signal power is split into the required wavelength bands, and then subjected to a predetermined weighting function.
The invention further resides in a broadband optical communications link including at least one of these amplifier arrangements.